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KMS kills 2D performance all across the board. However, it's not that bad with older chips (my 9800pro and 200m for example, and I would guess any pre r5xx chip as well); with those it remains rather usable. With my hd4850 KMS really screws things up (I will assume it's the same for any r6/7xx chip) as it is much much slower than UMS and certain applications are just unusable (Firefox being one).

Maybe that's because 2d runs on the 3d-engine on those newer chips?
With my x1900xt 2d seems very snappy, but I've only tested KDE-apps, not firefox or anything else.

On my RS780 board, which is basically a HD3200 board, I don't have any slowdown and Firefox works without a glitch.
Either there is something wrong with R700 based-boards (like the 2D made by the 3D engine) or there is something wrong with one of the stack of X provided by your distro.

If I compare my desktop system (rv770) with my rather ancient notebook (r300),
both on xorg-server 1.7.5, KMS, KWin (4.4), Linux 2.6.33 with mesa, drm and ddx from git,
the notebook performs perfectly smooth while the desktop system feels not as snappy.

Sounds plausible that recent chips (r600+) with a unified architecture require more
optimization work for 2d acceleration than the "older" parts that feature fixed function 2d hardware
that is now implemented using shaders. So it's (at least) one more layer, right?

On my RS780 board, which is basically a HD3200 board, I don't have any slowdown and Firefox works without a glitch.
Either there is something wrong with R700 based-boards (like the 2D made by the 3D engine) or there is something wrong with one of the stack of X provided by your distro.

The RS780 is like the rest of the R600+ (HD2xxx and up) in that it doesn't have a 2D engine either. Based on the comments here (most 6xx/7xx seem to be slower but not RS780 IGP) my first guess at the bottleneck would be less acceleration of transfers between system and video memory on R600+, since IIRC some of the 2D accel hardware on 5xx and below is also used for upload/download acceleration. Just a guess though.

My HD4890 PCIe system is running fine on F12.

Originally Posted by Melcar

KMS kills 2D performance all across the board. However, it's not that bad with older chips (my 9800pro and 200m for example, and I would guess any pre r5xx chip as well); with those it remains rather usable. With my hd4850 KMS really screws things up (I will assume it's the same for any r6/7xx chip) as it is much much slower than UMS and certain applications are just unusable (Firefox being one).

I have KMS, DRI2 and compiz all running fine with my HD 4890 and F12. Full-screen video playback seems OK too. And now I've upgraded my kernel to 2.6.33 (and unearthed the new firmware!) my video playback is looking tear-free, even when playing "the birds" HD file.

I think my xorg-x11-drv-ati is from git, but my libdrm and Mesa are definitely from F12's repositories still.

[Mind you, the HD4890 is in a new quad-core i7 PC - maybe that's why I'm not noticing any performance drop?]